Aims
The aims of the present invention are to:                1. Improve the heat transfer performance of flexible conduit bearing a heat transfer fluid, without substantive reduction in the ability of the flexible conduit to be easily arranged manually in a customized heat transfer circuit, meaning a circuit with a pattern that can be varied, possibly with in situ adjustment, to meet a wide variety of heating or cooling needs.        2. Provide a heat transfer system for radiant heating or cooling or solar or geothermal heat exchange that combines high heat transfer efficiency, ease of customization and simplicity in manufacture, design, ordering, delivery, installation and maintenance.        
PEX Tube and its Applications
Cross-linked high-density polythene (PEX tube) is a common example of a flexible fluid conduit, widely used to conduct hot and cold water. PEX tube is up to 6× cheaper per unit length than more traditional copper tube (2012 figures), is easily configured, has a long life, resists freezing damage and is highly resistant to corrosion.
PEX tube is also used in heat transfer applications, most often in hot-water under-floor heating. In heat transfer applications that permit use of standard heat-exchange assemblies, such assemblies will tend to be made of materials of high thermal conductivity: usually metals. For example, radiant heating or cooling panels that are suspended from ceilings are usually of standard dimensions and manufactured in, for example, steel. PEX has a thermal conductivity that is 800× lower than copper and 100× lower than carbon steel. This disadvantage can be outweighed by the ability of PEX tube to be readily configured in-situ, allowing the use of custom tube layouts.
Examples of Heat Transfer Applications of PEX Tube are:                Radiant heating or cooling systems in which a tube carrying a heat-transfer fluid is fixed in thermal contact with a large surface area that acts as a radiant emitter or receiver respectively. Using a flexible tube, such a system can be customized in situ to suit living spaces of differing geometries and with differing heating or cooling needs.        Solar heat collection systems in which a tube carrying a heat-transfer fluid is fixed in thermal contact with a heat-collecting surface. Using flexible tube, a solar thermal panel can be constructed in situ to fit the geometry of a roof.        Geothermal heating or cooling system in which a tube carrying a heat-transfer fluid is fixed in thermal contact with plates or fins that transfer heat from or to a body of water. Using a flexible tube, a curved tube layout can be fixed to heat-conducting surfaces supported by a snap-together frame: the in-situ assembly is sunk in a geothermal pond.        Geothermal heating or cooling system in which a tube carrying a fluid is fixed in thermal contact with plates or fins that transfer heat from or to the ground. Using a flexible tube, a curved tube layout fixed to heat-conducting surfaces can be placed in trenches that are laid out according to the available space.        
Improving Heat Transfer Efficiency
The applications mentioned above all refer to space heating and cooling. This accounts for a significant fraction of national energy use. For example, in the US and the UK, residential and commercial space heating and cooling is estimated to consume around 20% and 25% of national energy respectively. Improving heat transfer efficiency in such applications is beneficial both in saving costs and in reducing carbon emissions.
Heat transfer efficiency is improved by reducing the overall resistance of the thermal path between the heat transfer fluid inside the tube and the medium with which heat is being exchanged. The effect of reducing thermal resistance is to:                Reduce the temperature drop for a given heat flux.        In the case of a heating system, allow a lower operating temperature of the heating medium.        In the case of a cooling system, allow a higher operating temperature of the cooling medium.        
System-level aspects of heat transfer efficiency are discussed later.
In the applications mentioned above, the tube is fixed to a thermally conductive heat-diffusing or heat-collecting surface that is much larger than the surface are of the tube. Heat transfer efficiency is improved by:                Fixing the tube so that substantively 100% of the exterior surface of the tube is in intimate contact with the conductive surface.        Ensuring that the contact between the exterior surface of the tube and the conductive surface has no voids ie reducing thermal contact resistance.        Providing sufficiently extensive contact between the exterior surface of the tube and the conductive surface so that any voids are effectively bridged.        
The first of these requirements can be met if heat exchange units are fully assembled before installation. For example, the tube can be sandwiched between shaped metal plates or embedded in a conductive layer such as graphite-loaded plaster.
Flexible tube configured in situ can also be embedded. For example, flexible tube used in under-floor heating is often embedded in a screed that is initially liquid and sets to a solid. Typically this screed is based on calcium silicate or calcium sulfate. These materials have the defect of poor thermal conductivity.
A widely used alternative is to fix the tube to heat-conducting plates, usually made of aluminum sheet, or to a rigid panel with an aluminum layer. The aluminum sheet is formed into channels. The rigid panel is routed or molded to form channels and an aluminum layer is attached either pre-formed into channels or the layer is pressed into the panel. The inner diameter of the channels matches the outer diameter of the tube and the tube is pressed into the channels. By using a channel with an omega-shaped cross-section, the tube is retained. But such a channel only provides direct thermal contact between the plate and around 55% of the external surface area of the tube.
The area of thermal contact can be increased by a further installation step in which a heat conducting strip is fixed to the plate, covering the channel and the tube within it. Such a strip can be applied to straight sections of tube in the channel, but is difficult to apply to curved sections of channel, especially if this curvature is variable. Such a strip would be especially difficult to apply to a coiled tube layout.
Thermal contact resistance between abutting surfaces is reduced when such surfaces mate exactly, are very smooth and are held together under pressure. These conditions are not met by the conventional arrangement of tube and channel: the requirement for a press-fit between tube and channel and normal variations in tube and channel diameter mean that there is an uneven interface between the outer surface of the tube and the conducting surface. A partial solution is to embed the tube in a heat conducting filler inside the channel and under the strip. Flexible silicone grout can be used. But silicone grout has thermal resistance, being typically over 60× less conductive than, for example, aluminum, and use of grout requires a further installation step, in which an exact amount of filler is placed in the channel or excess filler is scraped off and removed.
The steps required in this example demand a significant level of skill, the steps being:                Install the channel-bearing heat-conducting plates (for example, on an insulating layer on a subfloor).        Ensure the channels are clean, for example by blowing out debris.        Place a layer of thermal filler (liquid, gel or paste) inside the channels.        Press the flexible tube into the channels.        Place a layer of filler over the tube.        Fix a heat conducting flat strip over the channels and press down, removing excess filler.        Continue pressing down until the filler has set.        
The present invention describes a modification of flexible tube that enables construction of a heat transfer system in which:                Thermal filler in a channel is not required.        A strip over the channel is not required.        
At the same time, overall heat transfer performance is superior because:                Substantively 100% of the surface area of the tube is in contact with the heat collecting/diffusing surface.        Thermal contact resistance is low.        Full contact between tube and plate is maintained over tube sections of any curvature.        
The present invention also describes how a modified flexible tube can be applied in a radiant heat transfer system that does not rely on channel-bearing plates or panels.
Prior Art: Tube with Improved Heat Transfer
In an example of the present invention, metal wire is used to improve the heat transfer performance of flexible tube. Metal wire is commonly embedded in flexible hose to prevent crushing and kinking. Helical wire used, not for heat transfer, but for reinforcement of a flexible tube dates to the 1600s. A relevant patent for pressure-resisting fire-fighting hose—UK patent no 263—was granted to John Lofting in 1690. Other examples are:
Mulconroy et al, U.S. Pat. No. 992,516; Sundh, U.S. Pat. No. 1,179,577; Onaka, U.S. Pat. No. 3,526,692; Lawless et al, U.S. Pat. No. 3,791,415; Stent, U.S. Pat. No. 3,938,939; Kanao, U.S. Pat. No. 4,140,154; Kovacs et al, U.S. Pat. No. 4,860,798.
Flexible polymeric hose reinforced with an embedded helix of high tensile steel wire is widely commercially available. Such hose is available in a variety of polymers, including PVC and natural and synthetic rubbers. The embedded wire is not well-suited to improved heat transfer through the wall of the hose since high tensile steel, although excellent for reinforcement, is a relatively poor thermal conductor, having, for example, around 16% of the conductivity of high purity aluminum. High purity aluminum, on the other hand, is not a good choice for reinforcement. Also, because the wire is embedded in the wall of the hose, it does not present a surface that can make direct contact to a heat-diffusing or heat-collecting surface of the kind that is required in a heat transfer system such as a system for radiant heating or cooling.
Hose with an exposed wire sheath—usually braided wire—is commonly used for mechanical protection and is also mentioned as means of preventing static build-up: for example: Kanao, US 2011/0247714.
Chiles (U.S. Pat. No. 4,779,673) describes a flexible, reinforced, multilayered hose that is embedded in concrete slab and conducts a heat transfer fluid. Applications include melting of snow, heating of buildings and transfer of solar heat. Both the inner and outer walls of the hose are made of polymer and the reinforcement is a layer of fabric braid. Improved heat transfer is claimed if the inner and/or outer layers incorporate thermally conductive fillers such as graphite or powdered metal. Typically, such filled polymers have feeble conductivity compared with continuous metal, having, for example, less than 2% of the conductivity of high purity aluminum. In the applications envisaged by Chiles, improved conductivity of the exterior hose wall would have little overall effect since the concrete slab would provide the controlling thermal resistance.
Jinbo (CN 201706080) describes heat-conducting helical wire either partly or completely embedded in a polymer tube. The aim of this is to both reinforce the tube and to improve heat transfer. The effect on the flexibility of the tube is not discussed. The partly embedded wire presents a highly conductive surface that can be brought into direct contact with a heat-diffusing or heat-collecting surface. The fully embedded wire does not have this property and is of less interest and, in any case, resembles known prior art.
In general, embedding of wire in a smooth-walled tube leads to reduced flexibility, affecting the ability to customize the layout of the tube. The effect on flexibility depends on the material of the wire and of the tube, the dimensions of the wire and of the tube, and the density of turns of the wire ie the % coverage by the wire of the area of the tube wall.
The most common relevant tube material is PEX and Jinbo refers to wire that is aluminum or copper. The modulus of elasticity of these metals is 80-150× the modulus of elasticity of HDPE, so that if we consider a layer of PEX with embedded wire, any stretching or compression in that layer will be concentrated in the PEX between the wire. As the density of turns increases, heat transfer performance improves because (a) a greater area of polymer tube wall is in intimate contact with a conductive surface, (b) a greater area of this conductive surface can itself (in relevant heat exchange applications) be in contact with a heat-diffusing or heat-collecting surface, and (c) overall contact resistance in the case of (b) is reduced because the turns provide bridges across areas of poor contact. But, at the same time, as the density of turns increases, the area of polymer between embedded turns shrinks, reducing the area available to stretch or contract when the tube is bent. As a result, the tube becomes stiffer. For example, if the surface area of tube wall covered by embedded wire rises to 50%, then the tube becomes approximately twice as stiff. As the density of turns is increased, custom layout becomes impractical.
In general, embedding of helical wire in smooth-walled tube requires the thickness of the tube wall to be increased and this also results in greater stiffness of the tube. It is obvious that a sufficiently fine wire will not require a significantly thicker tube wall. However, the finer the wire, the less effective it is as an agent for improved heat transfer. In the case of a standard size of PEX tube, the total outer diameter is 16 mm and the overall thickness of the wall is 2 mm. The wall comprises layers from the inside to the outside: PEX 1 mm, adhesive 0.1 mm, aluminum 0.2 mm, adhesive 0.1 mm, PEX 0.6 mm. If, to provide effective improvement in heat transfer, the desired diameter of embedded aluminum wire is 1 mm, then an additional thickness of wall is required to avoid compromising the integrity of the structure of the tube. If the wire is fully or partly embedded, this additional thickness is around 1 mm and 0.5 mm respectively, representing 50% and 25% increase in wall thickness.
In summary, if helical heat-conducting wire is embedded in flexible smooth-walled polymer tube, the tube becomes stiffer and less easy to configure in a custom layout. If very fine wire is embedded at a low density of turns, the effect on tube stiffness is small, but improvement in heat transfer is also small. If the wire is made thicker and the density of turns is increased, heat transfer improves but the tube with embedded wire becomes stiffer and eventually cannot be used in a custom layout.
In the present invention, this problem is solved.
Efficient Radiant Heat Transfer Systems
Hydronic (water-based) radiant heat transfer systems using PEX tube are increasingly used to heat and cool living and working spaces. The efficiency of such systems increases as the operating temperature falls (for a heating system) or rises (for a cooling system).
Heating efficiency rises with lower operating temperatures because:                If the heat source is a boiler, lower water return temperatures enable better heat recovery from combustion gases, with an efficiency gain as much as 25%.        If the heat source includes geothermal energy, the COP (coefficient of performance) of a heat pump increases by around 3% for every 1° C. drop in the temperature of the pump outlet.        If direct solar heating is used, lower return temperatures in the heat-transfer medium enable a flat plate solar collector to be used rather then a more expensive evacuated tube collector. At low return temperatures, a flat collector is significantly more efficient than the evacuated tube. As the return temperature rises above the ambient temperature, a flat collector radiates heat back into its surroundings faster than an evacuated tube. When the temperature difference (between return temperature and ambient temperature) exceeds around 40° C., the efficiency of the flat plate falls below the efficiency of the evacuated tube. In a representative design of flat plate solar thermal collector, over the normal range of operation, each 1° C. reduction in operating temperature results in around 1% improvement in collector efficiency.        
Lower operating temperatures are intrinsic in under-floor heating systems compared with other heating systems because under-floor systems deliver a near-optimum vertical temperature profile (warm feet and cool head). This profile enables greater comfort at lower temperatures.
Lower operating temperatures are achievable in any hydronic radiant heating system and higher operating temperatures are achievable in any radiant cooling system by means of:                A larger radiant surface. A larger surface can transfer heat at a given rate at a lower radiant surface temperature.        A more uniform radiant surface temperature. The surface temperature is made more uniform by reducing the thermal resistance of a layer on the thermal path in the plane of the radiant surface: the heat-diffusing layer (or in a cooling system, the heat-collecting layer).        Lower thermal resistance through the entire thermal path between heat source and radiant-emitting surface (for a heating system) or between a heat sink and a radiant collecting surface (for a cooling system). This enables heat to be transferred at a given rate at a lower temperature of heating medium for a given surface temperature.        Lower thermal mass and faster, zoned controls. This enables the system to respond more rapidly to local changes in required heating (or cooling) load, avoiding periods of unnecessary heating (or cooling).        
In addition, the ideal space-heating or space-cooling system:                Is cheap to manufacture, install and maintain.        Is easily adapted to different physical layouts and to different heating or cooling loads.        
Is unobtrusive.                Provides a high level of comfort.        
Current Practice in Radiant Heating
Historically, hydronic space-heating in Europe has used wall radiators. Such radiators heat mainly by convection, resulting in currents of warm air moving to the ceiling. This is the opposite of the temperature profile that is ideal for comfort. Wall radiators have limited radiant area and have to be run at higher temperatures, meaning lower system efficiency. Typical wall radiator systems require circulating water temperatures around 60-75° C. Operating temperatures for a typical under-floor radiant system are around 30-35° C. The aesthetics of wall radiators are generally poor: wall radiators are obtrusive and impede the arrangement of furniture.
The defects of wall radiators can be remedied by under-floor radiant heating. The temperature distribution through the living space is almost ideal for comfort. The entire floor becomes the radiating surface so that the system can run at lower temperatures. The system is invisible and the arrangement of furniture is not impeded.
Hydronic under-floor heating has been widely adopted in Europe. For example, in Germany around 75% of new build homes use this form of heating. In the UK, hydronic underfloor heating is only 5% of the domestic heating market but is growing rapidly.
In the USA, space-heating has been strongly influenced by the wide use of air-conditioning. Fans and ducts installed for air-conditioning can also be used for air-heating. As a result, forced-air heating is used in around 70% of US homes, usually with a gas furnace as heat source. The forced air system is relatively cheap in newly-built structures, provides a fast response and the visual aesthetics are acceptable. The main defect is poor comfort: the temperature distribution is uneven. There is also noise and the potential spread of allergens. Zone control is less effective. Compared with radiant hydronic systems, energy efficiency is reduced by heat losses in the ducts, by the need to use higher operating temperatures to compensate for poor temperature distribution and the lack of fine zone control. If a renewable heat source is used, this requires a liquid/gas heat exchanger and efficiency is reduced by thermal resistance on the gas side.
PEX tube is used in under-floor radiant heating by fixing the tube in a serpentine or coiled layout in the plane of the floor. A number of different methods are used:                Attaching the tube to conductive heat-diffusing plates (usually aluminum) that are fixed between joists under a subfloor. The underside of the plates is insulated. This is a retrofit method. The large thermal resistance above the plates makes this method inefficient.        Fixing the tube above an insulating layer over the subfloor and burying the tube in a screed. The tube is fixed using channels in the insulating layer or using clips or using an embossed insulator (a castellated insulator) in which the tube can be pressed and held between protrusions. The screed acts as the heat-diffuser. This method is commonly used, but it is inefficient because the screed is a poor conductor and also has thermal mass. If screed is used above the ground floor, the building structure must have sufficient strength to bear the added load. Maintenance may require the solid screed to be hacked out with a jackhammer.        Fixing the tube inside an insulating layer over the subfloor. This is quicker than using screed and avoids the thermal mass of screed but upwards heat transfer is still poor.        Fixing the tube inside a conductive heat-diffuser mounted on an insulator over the subfloor. In one version of this solution, the heat-diffuser and the subfloor are combined. This solution provides a desirable combination of uniform heat, low thermal resistance and low thermal mass. The tube is fixed into channels in the heat-diffuser. The channels have an omega-shaped cross-section so that the tube is retained.        
The last-mentioned solution is still defective:                The thermal resistance between hot water and heat-diffuser is significant: only around 55% of the tube is in contact with the spreader.        The mechanical contact between tube and heat-diffuser is uneven, resulting in significant thermal contact resistance.        The heat-diffuser is typically a standard thickness so that it is over-sized for high heat loads and under-sized for low heat loads. Heat loads will vary from room to room, for example, with higher heat loads in high-ceiling rooms with many windows and lower loads in interior corridors.        The channel-bearing panels used are typically a standard size, providing fixed spacing between tube runs. This means that the heat flow rate per unit area does not vary across a room, except as a result of falling water temperature as heat is given up. Yet some parts of a room have a higher heat demand than others. For example, closer tube spacing may be needed under a large window or under a high ceiling or may be desired in a bathroom. Wider tube spacing may be sufficient close to an interior wall.        The channel system requires panels with straight and curved channels to be laid side by side so that the tube runs in a continuous serpentine from a manifold and back to a manifold. The right number of each kind of panel must be computed and ordered. The need for exact customised design puts up the cost of such a system both in design and logistics.        In many channel systems, heat-diffusing plates are available only for the straight portions of the tube layout. By avoiding curved channels in the heat-diffuser, manufacturing is simplified. However, the radiant area is reduced.        Where the heat-diffuser and subfloor are combined, the heat leakage downwards is significant because standard subfloor material (for example, oriented strand board) is not a very effective insulator: the thermal conductivity of engineered wood is typically 4 to 5× the thermal conductivity of foam polymer.        
The present invention offers a solution to these defects.
Prior Art: Radiant System
Specific examples of prior art follow.
Becker (U.S. Pat. No. 2,799,481) describes extruded metal floor panels with integral upward-facing channels of U-shaped cross-section in which heating pipe is laid. Metal strips fit over the channels. This design does not accommodate the curved sections of the tube and does not accommodate variation in tube spacing. There is significant contact resistance between tube and strip and between strip and panel.
(U-shaped channels with a slightly curved lip are also called here omega-shaped channels, or, sometimes in the patent literature, C-shaped channels).
Jacobsen (EP 0094953) describes floor panels that combine a load-bearing function with a heating function. The panels comprise a lower load-bearing layer, a middle insulating layer and an upper heat-diffusing metal layer. Upward-facing channels in the panels with a U-shaped cross-section hold tubes carrying a heat transfer fluid. This design is deficient as follows: (a) only around 50% of the surface are of the heating tube is in contact with the heat-diffusing surface, (b) only straight troughs are used, so that curved sections of tube are not in contact with the heat-diffusing surface, (c) the radiant system must be custom designed exactly in advance of construction and the components must be delivered accurately. In practice, errors are made and decisions changed, incurring cost and delay, (d) to avoid complexity, the panels must be restricted to a limited range of sizes so that variation in heating needs cannot be matched by variation in tube spacing.
Bourne (U.S. Pat. No. 4,782,889) describes a radiant floor heating system comprising a metal deck placed over the floor joists. Attached to the underside of the deck are metal troughs with a U-shaped cross-section. The deck and troughs support the structural load. Heat transfer tubing is snapped into the troughs to form a heating circuit. (To allow easy customization of the tube layout, the tube has to be flexible ie polymeric). Heat is diffused through the deck. The same comments apply as for Jacobsen above.
Pickard (U.S. Pat. No. 5,454,428) describes a radiant heating panel comprising an extruded aluminum plate with an integral channel of U-shaped cross-section into which flexible heating tube can be snapped. An array of plates is arranged so that the heating tube forms a circuit and the plates are supported on wooden sleepers on a subfloor with the channels facing downwards into gaps between adjacent sleepers. The comments made above re Jacobsen also apply here.
Grant (U.S. Pat. No. 5,743,330) describes a similar extruded aluminum plate and the same comments apply.
Fitzemeyer (U.S. Pat. No. 6,283,382) describes a similar extruded aluminum plate that can include a heat-conducting cap over the groove. However, there will be significant contact resistance between tube and cap and between cap and plate. This problem is only partly solved by using thermal grout in the channel.
Fiedrich (U.S. Pat. No. 5,579,996) describes a heat-diffusing plate that carries a heating tube in a channel with a U-shaped cross-section. The plate is supported by an insulating panel so that the channel faces upwards. Thermal contact between channel and tube is improved by embedding the tube in a thermal grout. The same comments apply.
Other similar radiant heat transfer systems are described by, for example:
Fennesz U.S. Pat. No. 4,646,814; Shiroki U.S. Pat. No. 4,865,120; Bilotta, U.S. Pat. No. 5,743,330; Alsberg U.S. Pat. No. 5,788,152; Muir, U.S. Pat. No. 6,533,185; Sokolean U.S. Pat. No. 6,910,526; Kayhart, U.S. Pat. No. 7,832,159; Stimson U.S. Pat. No. 7,939,747; Fiedrich US 2004/0026525; Newberry US 2008/0264602; Andersson US 2009/0314848; Ross US 2011/0052160;
In all the instances given above, heat transfer would be significantly improved by inserting into the U-shaped (or omega-shaped) channels the flexible tube with improved heat transfer, as described in the present invention.
Prior Art: Castellated Panels
Castellated panels allow a heating circuit to be laid out in any desired configuration. Such panels are routinely used to lay out flexible hydronic heating tubing on subfloor, prior to covering with screed. Typically, the panel not only holds the tubing but also has an insulating and load-bearing function.
Feist (U.S. Pat. No. 4,250,674) describes an array of interlocking castellated panels that holds flexible heating tube in any desired curved pattern. Heat-diffusing metal plates are fastened by screws to the tops of the castellations. However there is no direct contact between the heating tube and the metal plate so that heat transfer is poor.
Hagemann et al (U.S. Pat. No. 4,640,067) describe a castellated mat for under floor heating comprising a layer of insulation, covered by an abrasion-proof layer that is molded into a regular array of protrusions between which heating tube can be held. Once the tubing has been arranged in circuit, screed is poured over the assembly. This method provides poor heat transfer due to the low thermal conductivity of the screed and also causes inefficient heating control due to the thermal mass of the screed.
Fawcett et al (U.S. Pat. No. D541,396) and Stephan (U.S. Pat. No. D587,358) describe typical geometries for a molded castellated panel.
Adelman (U.S. Pat. No. 8,288,689) describes a castellated panel that has the additional feature of a thermally conductive layer covering the upper surface of the panel, so avoiding the need for screed. However, it would be difficult and expensive to shape metal sheet into the form of a castellated panel. Steel is sufficiently ductile to be pressed into complex curves but is a relatively poor conductor of heat. Aluminum, by contrast, is an excellent conductor of heat but insufficiently ductile. Adelman proposes that a metallic layer be applied to the panel surface by spraying or plating. However, such a layer would be too thin to be an effective heat diffuser. A critical deficiency is that the contact between heating tube and heat-diffusing surface is limited to significantly less than 50% of the surface area of the tube.
Backman (U.S. Pat. No. 8,020,783) describes a castellated panel that has the additional feature of a heat-diffusing panel that fits into guides in the castellated panel and is attached by screws to the castellations. Again the contact between heating tube and heat-diffusing surface is limited.
In an example, the present invention employs a castellated panel to enable freedom of tube layout and to simplify the process of designing, ordering, delivering and installing the radiant system, and, at the same time, solves the problem of poor heat transfer that arises in the instances described above.
Before fully describing the present invention, it is helpful to define what is meant by flexible tube.
Flexible and Elastic Tube
PEX tube is an instance of a class of flexible and elastic tubes of relatively low thermal conductivity that can be used to conduct a heat transfer fluid. The flexibility and elasticity of such tube allows it to be easily arranged in a tightly curved layout by manual methods, where such a layout can be, for example:                A flat meander or serpentine        A raised meander, for example, a meander on a cylindrical surface        A flat coil or spiral        A raised coil or helix.        
The flexibility of tubes can be defined by the bending radius (BR), meaning the minimum possible radius of the bent tube expressed as a multiple of the outer diameter of the tube. By definition, if the tube is bent to a radius smaller than the minimum, then the tube kinks. In general, BR depends on:                The extensibility and compressibility of the material of the tube.        The temperature of the material at the time of bending. For example, the bending radius of PEX is increased by a factor of over 2× if the temperature is reduced from 20 to 0° C. If the temperature is raised to 130° C., this is the softening temperature of PEX and the bend radius is greatly reduced.        The method of bending (incorporating sand or ball bearings or a snugly-fitting spring into the tube inhibits kinking).        Duration of bending.        The profile of the wall of the tube (smooth v corrugated)        The thickness of the wall of the tube.        Reinforcement of the tube, for example by a metal helix or by braiding.        
In a typical domestic space heating application, using hot water conducted through a flat serpentine tube, the standard spacing of adjacent turns of the tube requires that BR=15 (approximately).
For soft copper in typical plumbing diameters, BR=3 to 4 so that copper can be configured in a suitable serpentine. However, copper is relatively stiff: having a modulus of elasticity of 105-120 GPa: this is 50-100× the modulus of elasticity of common polymers. As a result of its low elasticity, copper requires special bending equipment: creation of an extensive custom curved layout in situ is impractical.
The present invention concerns relatively elastic tube, where this can be defined as having a modulus of elasticity less than 2 GPa. Examples of modulus of elasticity are:
Rubber: 0.01-0.1 GPa
Low density polythene: 0.24 GPa
Polyurethane: 0.1-0.7 GPa
Polypropylene: 1.5 GPa
Nylon 6: 1.8 GPa
The present invention also concerns flexible and elastic tube that has low thermal conductivity where this is defined to be less than 2 W/m° C. Most common polymers have a thermal conductivity less than 1 W/m° C. For example, the thermal conductivity of PEX is around 0.45 W/m° C.
As an example, for PEX tube at 20° C., BR=6 and the modulus of elasticity of high-density polythene is around 0.8 GPa, so that the flat serpentine required for domestic space heating is easily achieved without special bending equipment. In cold conditions, the tube may need to be heated, for example, by using a hot-air blower. The temperature range of PEX is −100° C. to +110° C., making PEX suitable for a variety of heating and cooling applications.
As an example, for rigid poly vinyl chloride (PVC) tube at room temperature, BR exceeds 250 and the modulus of elasticity is around 3.2 Gpa. If the tube is warmed to around 40° C., BR is reduced to about 10 and elasticity is greatly increased. Therefore, by warming the PVC tube using simple methods (such as interior electrical heating), it is possible to construct a useful curved layout. The temperature range is 0 to 60° C. This narrow range limits the utility of rigid PVC in heat transfer applications. Chlorinated PVC has a wider range: from −40° C. to +90° C.
PVC can be made highly flexible by adding plasticizer. In an example, flexible PVC is extruded round a steel helix. In this configuration, BR=2 to 3, allowing a useful curved layout to be constructed. By using an inner liner of polyurethane, this kind of tube may be used for heating or cooling of consumable liquids. The temperature range is −20° C. to +90° C.
Flexible and elastic tube can also be constructed from a steel helix bonded to fiber-glass cloth with neoprene inner and outer layers. This tube has a temperature range of −50° C. to +150° C. Similar tube using silicone rubber inner and outer layers has a temperature range of −85° C. to +310° C. For both kinds of tube, BR=0.5.
There are many possible combinations of reinforcing helix and polymer, providing tubes that are flexible, elastic and able to serve over temperature and pressure ranges useful in heat transfer systems. Likewise, there are many possible combinations of reinforcing braid and polymer, providing tubes that can be used in heat transfer applications. Typical values of BR are in the range 2 to 10. For example:                PVC reinforced with nylon braid.        Silicone rubber reinforced with fiber-glass braid.        Neoprene reinforced with steel wire braid.        Polyurethane reinforced with polyester braid.        Nylon reinforced with steel wire braid.        
PEX tube may have multiple layers. For example, the layers in Al-PEX are in sequence from the inside of the tube: PEX, bonding agent, aluminum, bonding agent, PEX.
The aluminum layer prevents diffusion of oxygen through the walls of the tube. Typically the aluminum layer is created by folding a strip of metal round the inner PEX tube and by butt-welding this strip. A typical PEX tube of 16 mm OD has an aluminum layer of 0.2 mm inside a wall that is 2.5 mm thick. The aluminum is a ductile and malleable alloy that allows bending of the tube. Although the aluminum layer makes the tube stiffer than a tube made entirely of HDPE, it is still easy to create a desired curved layout manually. The preferred aluminum alloy is 3003 HO: low temper, high-purity aluminum (around 97% pure) with a small amount of manganese (1-1.5%) added. This alloy has a maximum elongation of around 20%, allowing a value of BR=5.